1. Field of the Invention
The present invention relates to a tape-form oxide superconductor and, more particularly, it relates to a tape-form oxide superconductor used for superconducting magnets, superconducting cables and electric power machines and instruments.
2. Description of the Related Art
Since the critical temperatures of oxide superconductors exceed the temperature of liquid nitrogen, they find application as superconducting magnets, superconducting cables and electric power machines and instruments.
In order to apply superconductors to the above fields, it is necessary to fabricate a wire having a high critical current density (Jc) and a long length. In order to obtain a long tape, it is necessary to form an oxide superconductor on a metal tape for mechanical strength and flexibility. Since oxide superconductors change their superconducting characteristics with changes in their crystalline alignment, it is necessary to improve in-plane alignment and it is therefore necessary to form an oxide superconductor on a tape-form, in-plane aligned substrate. In order to improve the critical current density, it is necessary that the c-axis of the oxide superconductor be aligned perpendicularly to the plane of the substrate and that its a-axis (or b-axis) have in-plane alignment almost parallel to the direction of current (hereinafter, referred to as xe2x80x9cc-axis alignmentxe2x80x9d and xe2x80x9ca-axis in-plane alignmentxe2x80x9d, respectively) so as to keep the quantum connectivity of a good superconducting state.
When an oxide superconducting layer is formed on a tape substrate by means of sputtering, pulse laser vapor deposition (PLD), vapor deposition or chemical vapor deposition using metalorganic salts (MOCVD), the crystal alignment of the substrate is usually as random polycrystals and, therefore, the oxide superconductor formed on this substrate is affected by the substrate and is unable to have a high degree of alignment.
Therefore, a method of using biaxially textured Ni (100) substrates as the tape-form substrates has been investigated. In this method, a nickel substrate is cold rolled and then heated in vacuo to make a highly aligned product called RABiTS (trade mark Rolling-Assisted Biaxially Textured-Substrates). There has also been reported a method wherein cerium is deposited on the biaxially textured nickel substrate in an atmosphere of inert gas at high temperature by means of electron beam evaporation. Hydrogen is present during this deposition whereby a thin epitaxial layer of CeO2 is formed, then a thick film of YSZ (yttria-stabilized zirconia) is formed thereon at high temperature in vacuo by sputtering and the resulting product is used as a substrate.
In the above method, a layer of YBCO (as used herein xe2x80x9cYBCOxe2x80x9d refers to the Yxe2x80x94Baxe2x80x94Cuxe2x80x94O type) is formed by pulse laser vapor deposition on the above-described substrate (refer, for example, to John Emathis, et al., Jpn. J. Appl. Phys., Vol. 37 (1988), pages L1379-1382).
The CeO2 layer on the biaxially textured Ni substrate is aligned so as to depress reaction of the Ni substrate with YSZ and to prevent oxidation of the Ni substrate which forms NiO islands on the nickel, while the YSZ layer is aligned so as to depress reaction with the superconducting layer. In other words, the YSZ layer functions as a buffer layer for preventing the diffusion of Ni, whereby reduction in the superconducting characteristic is prevented and the matching with the superconducting layer is maintained. With the biaxially textured Ni substrate, when the YSZ is directly deposited, Ni in the substrate reacts with Zr in the YSZ at their interface whereby no epitaxial growth takes place. When YSZ is directly aligned on the Ni substrate, the oxidation of the Ni substrate occurs whereby no epitaxial growth takes place. Accordingly, the YSZ is superimposed on the CeO2 layer which does not react with the Ni substrate, whereby diffusion of the element constituting the substrate into the superconducting layer is prevented. Since CeO2 is easily broken, a thick film of YSZ is formed on the thin film of CeO2.
In the above method, YBCO is formed on YSZ having a good matching with YBCO but, since CeO2 has a better crystallographic matching with YBCO than YSZ and further since CeO2 is also better in terms of reactivity with an MOD solution, in one reported method a thin film of CeO2 is further formed on the YSZ and a YBCO layer is formed thereon by MOD (metal organic deposition) to give a five-layered structure of biaxially textured Ni substrate/CeO2/YSZ/CeO2/YBCO (A. P. Malozemoff, et al., Eucas Conference, Sep. 14-17, 1999).
Metal organic deposition is a method where a metalorganic salt is applied and then thermally decomposed. Thus, a solution in which an organic compound having metal component is uniformly dissolved is applied on a substrate and heated for thermal decomposition whereupon a thin film is formed on the substrate. It does not require a vacuum and is able to achieve a high deposition rate at a low cost, whereby it is suitable for the manufacture of long tape oxide superconducting wires.
Since the MOD method uses a metalorganic salt as a starting material, it is also applicable to formation of an RE (123) superconductor, i.e. an RE1+XBa2xe2x88x92XCu3OY superconductor (as used herein, RE means Y, Nd, Sm, Gd, Eu, Yb or Ho) and to formation of an intermediate layer such as CeO2. When organic salts are thermally decomposed, alkaline earth metal (such as Ba) carbonate is usually produced and, since a high-temperature thermal treatment of 800xc2x0 C. or higher is necessary for the formation of an oxide superconductor by a solid state reaction of the carbonate, a method has been intensively used in recent years wherein an organic salt containing F (such as a TFA salt [trifluoroacetate]) is used as a starting material in a thermal treatment in a steam atmosphere, with control of steam partial pressure, whereupon an RE (123) superconductor is formed.
In that method where the TFA salt is used as a starting material, no nucleation results in a precursor and the RE (123) superconductor can be epitaxially grown on the substrate by the reaction of the steam with an amorphous precursor containing fluorine.
FIG. 8 shows a tape-form oxide superconductor 10 in a five-layered structure where CeO2, YSZ, CeO2and RE (123) superconducting layers are sequentially formed by the MOD method on the above-mentioned biaxially textured Ni substrate. In the drawing, there is shown a structure where a first intermediate layer 12 comprising CeO2, a second intermediate layer 13 comprising YSZ and a third intermediate layer 14 comprising CeO2 are formed on the biaxially textured Ni substrate 11 and, on the third intermediate layer 14, an RE (123) superconducting layer 15 is formed by an MOD method using a TFA salt.
In the above-mentioned tape-form oxide superconductor 10 with a five-layered structure, the CeO2 first intermediate layer, the YSZ second intermediate layer, the CeO2 third intermediate layer and the superconducting layer are epitaxially grown on a biaxially textured Ni substrate and, in addition, reaction among the elements constituting the biaxially textured Ni substrate and the superconducting layer is depressed whereby it is possible to prevent loss of the superconducting characteristic. In principle, the foregoing is an excellent method for the manufacture of a tape-formed superconductor.
However, with regard to the biaxially textured Ni substrate used in this method, it is necessary that a cold rolled Ni substrate be subjected to a thermal treatment in vacuo to give a high degree of orientation and there is the disadvantage that, during the recrystallization by such a thermal treatment, grain growth will take place to the extent of forming grains as big as 100 xcexcm or more and adversely influence the superconducting layer to the extent that the expected Jc value is not obtained.
Thus, at the (surface) grain boundary in the biaxially textured Ni substrate, disorder in the crystal structure occurs and, therefore, the intermediate layers and the superconducting layer formed on it are also affected, and such an influence becomes bigger especially when the growth directions of the adjacent crystal grains are greatly different.
Here the effect of the grain size on Jc of a long tape when the variation among the growth directions is constant will be considered. Jc of a tape material is determined at the area where Jc is lowest and, even if the variation in the growth directions is same, the result when the grain size is big is that numbers of grains aligned perpendicular to the direction of the current becomes small and the influence of the reduction in Jc value at grain boundaries having many growth directions on the Jc value as a whole becomes larger, whereby the Jc value is apt to become small. On the other hand, when the grain size is small, the numbers of the grains with a perpendicular alignment becomes large and the influence of grain boundaries on the Jc as a whole is nearly the same as that of the variation and does not cause a large reduction. Our studies obtained results wherein the larger the crystal size, the lower the Jc value.
In addition, since the biaxially textured Ni substrate is ferromagnetic and, further, its mechanical strength is low, when a superconducting magnet or superconducting cable is formed as a tape of an oxide superconductor, there is the problem that the influence of an external field increases to cause a reduction in the Jc value.
The present invention has as its objective, solution of the above-mentioned problems and, toward this end, provides a tape-form oxide superconductor having an excellent superconducting characteristic and being suitable for use as superconducting magnets and superconducting cable. The method of the present invention utilizes a metal substrate giving an improved quantum connectivity of the superconducting layer, i.e., improved Jc value due to high c-axis orientation and a-axis in-plane alignment, being non-magnetic or weakly magnetic and having high mechanical strength.
In order to achieve the above object, a tape-form oxide superconductor is produced by the present invention wherein:
on a tape-form metal substrate which is non-magnetic or only weakly magnetic and which has high strength, there are sequentially deposited:
an intermediate layer having a high degree of alignment, which functions to suppress diffusion of elements from the metal substrate into the superconducting layer and to suppress the reaction of the elements constituting the superconducting layer, the intermediate layer being formed by deposition of particles generated from a target onto the metal substrate with ion irradiation from a direction inclined with respect to the metal substrate; and
an oxide superconducting layer formed by applying a coating of metalorganic salts followed by thermal decomposition.
In another aspect of the present invention the above intermediate layer is formed as a two-layered structure. This tape-form oxide superconductor is produced as follows:
on a tape-form metal substrate which is non-magnetic or only weakly magnetic and which has high strength, there are sequentially deposited:
a first intermediate layer having a high degree of alignment, serving to suppress the diffusion of elements from the metal substrate into the superconducting layer and to suppress reactions of the elements constituting the superconducting layer formed by deposition of particles generated from a target onto a metal substrate, with ion irradiation from a direction inclined with respect to the metal substrate,
a second intermediate layer having a good matching with the oxide superconductor, and
an oxide superconducting layer formed by applying a coating of metalorganic salts followed by thermal decomposition.
According to the present invention, an intermediate layer is formed by deposition of particles generated form a target onto a substrate together with ion irradiation of the metal substrate and, therefore, it is possible to directly form a YSZ layer or film, etc. having good alignment on the substrate. The above-mentioned formation of NiO on the substrate is prevented and it is no longer necessary to coat the YSZ via a CeO2 layer which does not react with the substrate.